CHAPTER 8

A Second Look at Classes

and Objects

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A note about coverage

• Say you have an if statement:

if (sunShining && temp > 75)

• You need 4 tests. sunShining T/F and temp > 75 and temp <= 75.

• If you can’t make 4 tests, you probably have something ahead that is making one of the conditions obsolete.

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Quiz today

• Two parts– White paper – your individual answer – put these in

the folder when done. Make sure name is on.– Green paper – group responses – start only when

ALL of the individual papers are submitted.– Grades based on 40% individual effort and 60%

group. – 30 minutes total.

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Static Class Members

• Static fields and static methods do not belong to a single instance of a class.

• To invoke a static method or use a static field, the class name, rather than the instance name, is used.

• Example:

double val = Math.sqrt(25.0);

Class name Static method

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Static Fields

• Class fields are declared using the static keyword between the access specifier and the field type.private static int instanceCount = 0;

• The field is initialized to 0 only once, regardless of the number of times the class is instantiated.– Primitive static fields are initialized to 0 if no initialization is

performed.

• Examples: Countable.java, StaticDemo.java

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Static Fields

instanceCount field(static)

3

Object1 Object3Object2

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Static Methods• Methods can also be declared static by placing the static

keyword between the access modifier and the return type of the method.public static double milesToKilometers(double miles)

{…}

• When a class contains a static method, it is not necessary to create an instance of the class in order to use the method.double kilosPerMile = Metric.milesToKilometers(1.0);

• Examples: Metric.java, MetricDemo.java

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Static Methods

• Static methods are convenient because they may be called at the class level.

• They are typically used to create utility classes, such as the Math class in the Java Standard Library.

• Static methods may not communicate with instance fields, only static fields.